There have been developed an optical disc on which information can be recorded at different positions in the depth direction in its recording layer (See the document: K. Saito and S. Kobayashi, “Analysis of Micro-Reflector 3-D optical disc recording”, IEICE Technical Report, Vol. 106, No. 248, CPM2006-82, pp. 19-23, September 2006). In the optical disc, information is recorded at a position with the desired depth utilizing the fact that a micro reflector (micro hologram) with a diffraction limit size is formed by converging two opposing light beams at the same focal point in a volume type recording medium.
When recording and/or reproducing information on the optical disc according to the document, the recording and/or reproducing position along the tracking direction can be read out by emitting a light flux for a servo operation upon the reference layer formed in the optical disc. Further, the depth position of the recording layer can be found from the distance of the recording layer from the reference layer. However, if some external disturbances such as vibrations occur while the information is being recorded or reproduced, the light flux will jump to a different depth position, whereby a recording or reproduction error may occur. In order to avoid this error, it is possible to consider returning the light flux to the first reference layer and then searching for the depth position. However, there is the problem that information recording/reproduction gets interrupted during the search and it takes time until the information recording/reproducing is restarted. In addition, when the optical information recording and reproducing apparatuses to record and/or reproduce information for that optical disc have individual differences, some optical information recording and reproducing apparatuses record information at recording positions at minutely different depth due to the individual difference. Therefore, the distance from the reference layer will not always be the same among the optical information recording and reproducing apparatuses, which is also a problem. Furthermore, when an optical information recording and reproducing apparatus makes random accesses to the optical disc, the optical information recording and reproducing apparatus needs to verify the location where the information to be read has been written.